Cylinder blocks of two-cycle cranckase compression engines are normally fabricated by die casting. Recently, there has been increased activity in fabricating the cylinder block by a lost foam process in which the pattern is made from an evaporable foam material, such as polystyrene.
In the typical lost foam process, the pattern is placed in a mold and a flowable material, such as sand, is disposed around the pattern and within the cavities in the pattern. During casting, the heat of the molten metal will vaporize the pattern and the vapor will be dispersed in the interstices of the sand, while the metal fills the area vacated by the foam material.
The fabrication of an evaporable foam pattern for a two-cycle engine has proven difficult due to the complexity of the porting in the engine block and particularly the transfer ports. A typical two-cycle engine includes one or more transfer passages through which the fuel-air charge is drawn to the cylinder. The discharge end of the transfer passage constitutes a transfer port which must be positioned in precise tolerance with respect to the cylinder head so that the fuel-air stream will flow in the desired course to the head and then flow to the exhaust port to affect removal of the spent gases.
It has been proposed to fabricate a cylinder block pattern for a two-cycle in a series of sections, with each section being cut laterally through the axes of the cylinders. The sections are then glued together along mating interfaces to form the cylinder block pattern. By properly selecting the positions of the interfaces of the sections, it is possible to produce a pattern with the desired porting. However, as a cylinder block pattern of this type is formed of a group of glued up sections, it is difficult to maintain the proper tolerance between the transfer ports and the cylinder head, thereby increasing the machining cost for the die cast metal block. Further, any excess glue, which may extrude from the joints between pattern sections will generally extend transversely to the transfer passages and the extruded glued joint will result in a transverse bead of metal in the cast metal cylinder block which can adversely affect the flow of the fuel-air charge through the transfer passages.